The present invention relates to a surgical trocar for use in extending a cannula through the wall of a body cavity. In its more particular aspects, the invention is directed to a safety trocar having a piercing tip which extends distally from the cannula for purposes of forming a puncture in the wall and which, upon completion of the puncture, retracts into the cannula. The invention is especially concerned with an improved mechanism for sensing when the puncture is complete and triggering retraction of the piercing tip in response to this sensed condition.
Trocars are pointed surgical instruments which puncture tissue to obtain access to body cavities. Inherent with such instruments is the danger that after the intended puncture, the trocar will inadvertently puncture unintended tissue, vessels or organs.
The prior art teaches various techniques for providing needles and trocars with means to avoid inadvertent puncturing. For example, hollow Veress needles are provided with spring-loaded internal pins which pop out beyond the sharp cutting end of the needles after the intended puncture has been completed. U.S. Pat. Nos. 4,601,710 and 4,654,030 show trocars having shields in the form of a sleeve which extends around the point of the trocar after piercing is complete. The sleeves of the latter patents are received between the cannula being placed and the piercing point and, necessarily reduce the size of the piercing point which can be used with a cannula of a given diameter.
U.S. Pat. No. 4,535,773 teaches techniques for shielding the sharp tip of a trocar through either the interposition of an extendable shielding sleeve, or the retraction of the trocar into the trocar tube. The latter arrangement is seen in the embodiment of FIGS. 34 and 35 of the patent. It relies upon a solenoid operated detent which holds the trocar in the extended position relative to the cannula being placed and requires electronic sensing means in the piercing tip of the trocar to activate the detent for release.
More recent retracting tip trocars are found in U.S. Pat. Nos. 5,116,353, 5,152,754 and 5,158,552. The devices of the '353 and '552 patents employ movable piercing tips which retract partially during the piercing operation and, upon the completion of piercing, release for momentary distal movement and then retract fully into a shielded condition within the cannula. The device of the '754 patent employs a protective sleeve which extends through the cannula and is concentrically received around the piercing tip for partial retraction during the piercing operation. With the device of the latter patent, the sleeve retracts relative to the piercing tip in response to drag from the wall of the body cavity as the tip is advanced through the wall. Upon completion of the piercing function, the sleeve advances distally relative to the piercing tip to trigger retraction of the tip into the cannula.
The piercing tips in the devices of U.S. Pat. Nos. 5,116,353 and 5,158,552 "float" backwardly during the initial piercing operation and, upon completion of the operation, momentarily extend distally. This operation necessarily results in a tactile feel which is different than that which occurs in trocars wherein the piercing tip remains fixed during puncturing. It also, necessarily, requires that there be some extension of the piercing tip upon completion of the puncture. The device of U.S. Pat. No. 5,152,754 requires a shielding sleeve concentrically interposed between the piercing tip and the cannula being placed. The presence of this sleeve, necessarily, results in an increase in penetration force for a cannula of a given diameter.
It is desirable to minimize penetration force to enhance safety. For a given size cannula, penetration force can be reduced by maximizing penetrating head, or tip, size relative to the inner diameter of the cannula. This produces a larger cut upon penetration, thereby minimizing tissue dilation required to insert the cannula. Typically, the penetration force of existing devices is not optimized due to design constraints.